Driving with more passengers in a car, particularly young passengers, increases the number of people sneezing, coughing and spilling food, leaving a trail of germs behind that can get transferred to the hands and on to the nose, mouth, or eyes, leading to a potential infection.
"Children are little bags of germs and if you car them around, they have more potential pathogens," said Tierno.
Gerba put it another way: "If you're a soccer mom, you're essentially driving a germ-mobile."
Pointing out that bacteria can grow in food spills, Gerba recommends using disinfecting wipes in the car, especially on dashboards, door handles, and kid's car seats, especially when carpooling or transporting a lot of children.
These innocuous-looking offenders are difficult to avoid, which is part of the reason why push buttons can be crawling with germs.
Further, ubiquitous buttons, found on ATMs, elevators, telephones and drink machines, among other things, are located in areas that are not often cleaned and disinfected to kill bacteria and viruses.
Gerba noted that the first-floor buttons in elevators were the dirtiest.
"Everyone needs to go to the first floor," he said.
Worse, these germs get transferred to the body part that comes in contact with faces the most -- fingers and hands.
While avoiding these types of buttons can be almost impossible, Gerba does have a few recommendations.
"Knuckle it or wait for someone else to push it for you."
In some ways, cars are temporary living spaces that we often overlook as places where microorganisms can grow and thrive.
According to Hilton's study, the nastiest harbor for bacterial refugees was the carpet of the trunk, where scientists found 300 to 400 bacteria per square inch.
What is notable, Hilton said, was the diversity of activities people use their cars for -- dumping rubbish one day and carrying groceries the next. These differences make a car even more susceptible to a variety of pathogens.
Wiping surfaces and mats and vacuuming up crumbs and debris can be helpful for keeping germs at bay. Hilton also said you need to be more vigilant if your pet regularly joins you for a ride.
"These are probably the worst," Gerba said. "They are the germiest restrooms you'll run across."
But bathrooms in general are not as germ-ridden as other areas -- a kitchen sink or laundry machine, for example. Compared to several items on this list, toilets are a beacon of cleanliness because they are cleaned and disinfected on a regular basis, even public toilets.
Airplane bathrooms get cleaned, but the high volume of people they must cater to in a short amount of time leaves them very dirty very quickly.
Gerba said a normal aircraft has one bathroom per 50 people. Discount airlines have one bathroom per 75 people.
"There is a thin layer of E. coli over the sink," and other surfaces, Gerba said, adding that many people, especially men, will not wash their hands effectively because the sink is small, and dirty hands transfer germs to the face easily.
Interestingly, the cleanest toilets are probably those in public areas of a hospital, Gerba said.
"In a city, I'd pull into the emergency room."
You may be better off preparing your food on another surface.
According to Gerba, there are 200 times more fecal bacteria on a cutting board than a toilet seat.